1. Field of the Invention
The present invention relates to a method for tagging hydrocarbons which permits the presence of such hydrocarbons, especially as components of hydrocarbon mixtures, to be subsequently detected. The invention also relates to devices for detecting the tagged hydrocarbons and to fluorescent dyes appropriate for tagging hydrocarbons.
2. Description of the Prior Art
Hydrocarbon mixtures are, by their very nature, difficult to characterize and identify. Hydrocarbons are composed predominantly of carbon and hydrogen atoms, but can also contain relatively minor amounts of elements such as oxygen, nitrogen, phosphorus and sulfur. Aliphatic hydrocarbons consist of chains of carbon atoms that do not involve cyclic structures. Cyclic hydrocarbons are composed of atoms arranged in a ring or rings. Aromatic hydrocarbons are a sub-set of cyclic compounds having six membered rings which share hybrid carbon-carbon bonds. Other hydrocarbons are constituted by rings and chains, and may be aromatic to some extent.
Petroleum and associated natural gases are currently the major source of hydrocarbons. Petroleum is a complex mixture in which aliphatic, cyclic and aromatic hydrocarbons are present. Separation processes such as distillation and extraction separate crude petroleum into useful fractions, while conversion processes such as catalytic cracking create fractions not found in nature. For example, a gasoline fraction which can be recovered or manufactured from petroleum encompasses a range of C.sub.5 -C.sub.10 compounds having boiling points of about 40 to about 200.degree. C. Other principal fractions which can be recovered or manufactured from petroleum are kerosene which includes C.sub.8 -C.sub.14 compounds and has a boiling point range of about 175 to about 325.degree. C., gas oil which includes C.sub.12 -C.sub.18 compounds and has a boiling point above 275.degree. C., diesel fuel and lubricating oils.
Diesel fuel is defined for the present purposes as a hydrocarbon mixture having an initial distillation temperature of about 160.degree. C. and a 90% point, as determined by American Society for Testing and Material Test ASTM D86, of about 290.degree. to about 360.degree. C. An acceptable range for diesel fuel gravity specific density is about 0.8251 to about 0.8762. Flash point for diesel fuel is about 38.degree. C. or greater. Diesel fuel can often be utilized as fuel for heaters or turbines, as well as fuel for diesel engines. For the present purposes, heating oils and turbine fuels which meet the above-noted requirements for diesel fuel are considered to be diesel fuels.
Additionally, the characterization of many petroleum fractions is further complicated as the recovered fractions are blended with other fractions, chemically transformed or dosed with additives. Additives include octane enhancers, oxygenates, corrosion inhibitors and deposition inhibitors. Analytical procedures for detecting the presence of certain hydrocarbons in hydrocarbon mixtures are known. However, traditional analytical procedures are time-consuming and often provide only limited information about the mixtures.
The oil industry has recognized the need for a distinctive tagging agent or tracer which could be used to quickly and efficiently distinguish between seemingly identical mixtures of hydrocarbons, or, alternatively, to identify their manufacturing source or commercial destination. For example, a paper entitled "Identification of Petroleum Oils by Fluorescence Spectroscopy" by U. Frank was published in the Proceedings of the 1975 Conference on Prevention and Control of Oil Pollution, San Francisco, Calif., Mar. 25-27, 1975, pages 87-91. The Frank paper describes a method of passive tagging by fluorescence spectroscopy which involves excitation of petroleum oils at 15 wavelengths between 225 and 500 nanometers, at 20 nanometer intervals. The Frank paper states that the maximum emission intensities are plotted versus the excitation wavelengths to derive silhouette profiles used as fingerprints for passive tagging.
Passive tagging has been criticized, however, on the grounds that the naturally occurring compounds available for use as passive tags are not sufficiently identifiable and stable. As alternatives, three methods of active tagging are recommended in a paper entitled "Oil Tagging Systems Study" published by the National Technical Information Society in May of 1970 under NTIS Accession No. PB-195 283. Halogenated polycyclic aromatics, organometallics, and coded microspheroids were reportedly examined and found to show promise as active tags for oils.
U.S. Pat. No. 4,755,469 issued to Showalter et al. describes a method for tagging an oil so that it may be subsequently identified. The Showalter et al. patent states that a rare earth metal salt of a fatty acid can be incorporated into the oil and, thereafter, an oil suspected to contain at least a portion of the tagged oil may be analyzed for the presence of the rare earth metal. However, conventional techniques which analyze for the presence of rare earth metals are typically expensive and time consuming.
A method for determining the presence of one or more liquid hydrocarbons in a liquid hydrocarbon mixture is described in U.S. Pat. No. 4,278,444 issued to Beyer. The method reportedly involves adding to the hydrocarbon to be detected a minor amount of an alkylated isodibenzanthrone which can subsequently be detected by means of fluorescence spectroscopy. The Beyer et al. patent recounts a procedure in which alkylated isodibenzanthrone is excited to fluorescence by visible light and emits fluorescent light in the visible spectrum. Such a procedure appears to be less satisfactory for use in hydrocarbons which naturally possess a significant degree of visible color or contain a visible artificial dye.
U.S. Pat. No. 5,279,967 issued to Bode describes a hydrocarbon liquid identification and tracing system based on fluorescence, chromatographic separation and the presence or absence of substituted naphthalimides. According to the Bode patent, one or more of a homologous series of substituted naphthalimide dyes, which absorb and fluoresce light in the visible spectrum, can be added to a hydrocarbon fluid in varying amounts as labels. The Bode et al. patent lists an excitation range for gasoline of 350-440 nanometers and an emission range for gasoline of 450-550 nanometers, and describes the substituted naphthalimides which reportedly emit at 550 nanometers as having acceptable excitation and emission spectra for use with gasoline.
Subsequently, the labeled hydrocarbon fluid is passed through a separation device, such as a chromatograph, to separate the labeling compounds from the hydrocarbon fluids and to detect the presence or absence of the labeling fluids according to their separation and fluorescent characteristics. The necessity of providing a chromatograph or other separation device seemingly disqualifies the system described in the Bode patent from consideration for applications requiring portability or a speedy determination.
U.S. Pat. No. 4,141,692 issued to Keller discusses a method of marking fuels with chlorohydrocarbon or chlorocarbon tracers. The tracers reportedly can be detected by gas chromatography using a pulsed electron capture detector. The method is said to be applicable to gasolines, diesel fuels, jet fuels, furnace oils, and kerosenes. Practitioners will appreciate, however, that the use of such chlorine containing tracers in fuel tends to increase the rate of metallic corrosion in associated engines or burners and also to increase the level of objectionable pollutants in combustion products.
The term dye refers to compositions of matter exhibiting absorption peaks at reproducible wavelengths. Fluorescent dyes are defined for the present purposes as compositions of matter exhibiting absorption peaks at reproducible wavelengths and, thereafter, emitting fluorescence radiation.
Known fluorescent dyes include, for example, polymethines. One such polymethine, 3,3'-diethyl-2,2'-(4,5', 4', 5'-dibenzo) thiatricarbocyanine iodide (hereinafter, "DDTC"), is described in an article entitled "Semi-Conductor Laser Fluorimetry in the Near-Infrared Region" by Imasaka et al., Analytical Chemistry, 1984, 58, 1077-1079 (June 1984). The article states that DDTC exhibits strong absorption in the region of 786 nanometers and fluoresces at approximately 840 nanometers when dispersed in methanol or benzene. The article also states that DDTC was less soluble and completely non-fluorescent in water. A procedure in which a semi-conductor laser fluorimeter was utilized to demonstrate trace analysis of surfactants based on ion-pair extraction of DDTC is reported.
An infrared absorbing phthalocyanine compound having organic substituents linked to at least 5 of 8 specified peripheral carbon atom positions is described in U.S. Pat. No. 4,606,859 issued to Duggan et al. The phthalocyanine compounds are said to absorb in the range 750 to 1100 nanometers. The Duggan et al. patent states that the organic substituents may be aliphatic, alicyclic or aromatic. The Duggan et al. patent proposes the phthalocyanine compound for use in absorbing electromagnetic energy from an infrared source, as in use with infrared inks or welding goggles.
Hydrocarbon-soluble, metal-containing naphthalocyanine compounds are reported in European Patent Application 84108500.4 which lists Tsunehito as inventor. The Tsunehito application states that the naphthalocyanine compounds are soluble in organic solvents and exhibit strong absorption of near-infrared radiation in the range of 750 to 850 nanometers. The application lists absorption peak wavelengths and absorptivity coefficients for several metal-containing naphthalocyanine compounds, but is silent on the subject of fluorescence. The described naphthalocyanine compounds have straight chain or branched chain alkyl groups, each of the alkyl groups having 5 to 12 carbon atoms.
Squarylium dyes, also called squaraines, are being marketed as near infrared photoreceptors for laser printers, according to an article entitled "Near-Infrared Absorbing Dyes," Chemical Review, 1992, No. 6, 1197-1226 (July 1992) by Fabian et al. The Fabian et al. article states that a symmetrical Squarylium exhibits photoconductivity at 830 nanometers. However, the Fabian et al. article is silent on the subject of fluorescence for Squarylium dyes.
Methods for tagging and for detecting and separating thermoplastic containers using near-infrared fluorescing compounds are proposed in International Application No. PCT/US92/08676 which lists Cushman et al. as inventors. The method for detecting and separating thermoplastic containers reportedly includes exposing a mixture of thermoplastic containers to near-infrared radiation having wavelengths of about 670 to about 2500 nanometers, detecting emitted fluorescent light, and separating the fluorescing containers from non-fluorescing containers by mechanical means. FIG. 1 of the Cushman et al. application depicts an apparatus for identifying thermoplastic polymers containing a near-infrared marker.
The Cushman et al. application reports that phthalocyanines, naphthalocyanines, and squaraines are useful as markers. Significantly, the Cushman et al. application states that the near-infrared fluorescing compounds must be thermally stable and suitable for admixing or copolymerizing with condensation polymers. Moreover, the Cushman et al. application notes that the markers may be employed as copolymers to produce marked thermoplastic compositions in which the marker is not readily separated.
A portable fluorescence instrument including ultraviolet excitation optics and fluorescence spectral optics is described in U.S. Pat. No. 4,301,372 issued to Giering et al. The fluorescence instrument reportedly includes an ultraviolet radiation source, such as a low-pressure mercury lamp. The Giering et al. patent states that the ultraviolet radiation is absorbed by most aromatic hydrocarbons which fluoresce. A fluorescence spectrum of the sample under examination is reportedly recorded by the instrument.
Despite the significant achievements of previous practitioners, a need still exists for an improved system of marking hydrocarbons and determining their presence. Preferably, the improved system includes a marker which is invisible to the naked eye, yet detectable by a relatively quick and simple test procedure. Preferably, the test procedure requires minimal instrumentation and creates no waste products for disposal. Desirably, the improved system permits detection of the marked hydrocarbon at comparatively low concentrations in mixtures with other hydrocarbons.
The limitations of traditional marking methods are perhaps best illustrated by reference to a problem currently facing gasoline regulators, producers, transporters and retailers. Reformulated gasoline conforms to stringent standards intended to reduce air pollution associated with automobiles. To achieve the benefit of reduced air pollution, the sale of conventional gasoline may soon be prohibited in certain areas of the United States. Enforcing such a prohibition is problematical because the visual appearance of reformulated gasoline and conventional gasoline is quite similar. Therefore, the Environmental Protection Agency of the United States Government (hereinafter, "EPA") is currently evaluating gasoline markers to assist in distinguishing reformulated gasoline from conventional gasoline.
The EPA's published criteria for conventional gasoline markers are cost-effectiveness, wide availability, stability, minimal environmental side effects and enforceability. Factors which affect the stability of the marker include the extent to which the marker forms deposits on gasoline-powered engines, the extent to which the marker interacts with gasoline additives, and the solubility of the marker in water (which is sometimes present in gasoline storage tanks). One factor affecting environmental side effects is whether detection of the marker creates waste products for disposal. Similarly, gasoline markers which contain metals, sulfur or chlorine tend to produce combustion products having adverse environmental side effects.
None of the previously known gasoline marker systems satisfy all of the EPA criteria. Moreover, of five conventional gasoline markers which the EPA is currently evaluating for use in distinguishing reformulated gasoline from conventional gasoline, one must be administered at dosages which form solid deposits in gasoline-powered engines. Another marker being evaluated reacts with detergent additives commonly employed in gasoline. The other three markers being evaluated are detected by means of chemical detection kits which produce waste materials necessitating disposal. These shortcomings are not oversights but, rather, limitations imposed by current marker technology. In light of these shortcomings, it is believed that an improved system for marking gasolines would be welcomed by government, industry, and consumers.